1. Field of the Invention
The present invention relates to the production of thin films. In particular, the invention concerns a method of growing a thin film onto a substrate, in which method the substrate is placed in a reaction chamber and it is subjected to surface reactions of a plurality of vapor-phase reactants according to the ALD method to form a thin film.
2. Description of Related Art
Conventionally, thin films are grown out using vacuum evaporation deposition, Molecular Beam Epitaxy (MBE) and other similar vacuum deposition techniques, different variants of Chemical Vapor Deposition (CVD) (including low-pressure and metallo-organic CVD and plasma-enhanced CVD) or, alternatively, the above-mentioned deposition process based on alternate surface reactions, known in the art as the Atomic Layer Deposition, in the following abbreviated ALD, formerly also called Atomic Layer Epitaxy or “ALE”. Commercially available equipment and processes are supplied by ASM Microchemistry, Espoo, Finland, under the trade mark ALCVD™.
In the MBE and CVD processes, besides other variables, the thin film growth rate is also affected by the concentrations of the starting material inflows. To achieve a uniform surface smoothness of the thin films manufactured using these methods, the concentrations and reactivities of the starting materials must be kept equal on one side of the substrate. If the different starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, the possibility of mutual reactions between the reagents is always imminent. Herein arises a risk of microparticle formation already in the in feed lines of the gaseous reactants. Such microparticles generally have a deteriorating effect on the quality of the deposited thin film. However, the occurrence of premature reactions in MBE and CVD reactors can be avoided, e.g., by heating the reactants not earlier than only at the substrates. In addition to heating, the desired reaction can be initiated with the help of, e.g., plasma or other similar activating means.
In MBE and CVD processes, the growth rate of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the thin film growth rate in the ALD process is controlled by the substrate surface properties, rather than by the concentrations or other qualities of the starting material inflows. In the ALD process, the only prerequisite is that the starting material is provided in a sufficient concentration for film growth on the substrate.
The ALD method is described, e.g., in FI Patents Nos. 52,359 and 57,975 as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI Patents Nos. 97,730, 97,731 and 100,409 are disclosed some apparatus constructions suited for implementing the method. Equipment for thin film deposition are further described in publications Material Science Report 4(7), 1989, p. 261, and Tyhjiötekniikka (title in English: Vacuum Techniques), ISBN 951-794-422-5, pp. 253–261.
In the ALD method, atoms or molecules sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon.
According to the conventional techniques known from FI Patent Specification No. 57,975, the saturation step is followed by a protective gas pulse forming a diffusion barrier that sweeps away the excess starting material and the gaseous reaction products from the substrate. Intermixing of the successive reactant pulses must be avoided. The successive pulses of different starting materials and the protective gas pulses forming diffusion barriers that separate the successive starting materials pulses from each other accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials.
As known in the state of the art, dosing of precursors with a high vapor pressure, e.g. TMA and H2O, makes it possible to use valves, which are operated at ambient temperature. As explained in our earlier patents the “inert gas valving”, comprising a diffusion barrier and a drain operated at ambient conditions, has made it possible to use the ALD process with these high vapor pressure materials. In the following, inert gas valving will be also referred to in the abbreviated form “IGV”. It is disclosed and discussed in more detail in our co-pending U.S. patent application Ser. No. 09/835,931 filed on 16 Apr. 2001, the content of which is herewith incorporated by reference. Today there is a growing interest for the use of low vapor pressure solid precursors. The source temperatures can rise up to above 500° C. This is the case for, e.g., MnCl doping of ZnS phosphors. This puts stringent demands on the valves employed for controlling the dosing. Also the use of the IGV is somewhat complicated due to solid condensation of the precursor in the ambient operated drain capillary, which can become blocked and by that ending the proper function of the IGV, causing failure of the process. In single wafer cluster systems, down time for cleaning of drain capillaries and replacing source tubes after each run is not acceptable, contrary to the situation for batch type multi layer processing of thin film displays.
Due to the cyclic nature of ALD processing conventional valves cannot be solo adapted for this kind of a process. The aggressive source media destroy the valve rapidly at such conditions.
In the low vapor pressure dosing system for ALD systems, the cyclic injection of a precursor into an ALD process requires a valve controlling the dosing. A mean time between failure of 20–40 million cycles would be preferable for such a valve for production reasons. In the earliest ALD reactor constructions, solenoid-type valves were mainly used employing valve seal materials of different kinds of elastomers or polymers. Later on, pneumatic valves with metal membranes and metal seats have been used. When aggressive precursors are used at elevated temperatures, involving continuous closing actions, the result is rapid wear of the valve seal. Even pneumatically activated metal membrane valves release metallic particles into the process flow at such conditions. A solenoid valve often creates abrasive products as a result of the steam grinding the solenoid housing. For purity and safety reasons small, convenient sized solenoids are not preferred in chemical vapor deposition equipment.
Mass Flow Controllers (in the following abbreviated “MFC”) are widely used for controlling the precursor dosing into conventional (not pulsing) CVD systems but they cannot be used for fast pulsing ALD systems due to their slow response (long response times). At ambient conditions, pneumatic valves can tolerate only 0.2–4 million cycles due to wear out of actuator seals and deformation of the valve steam and valve seat. This is because of forces acting on the valve. At elevated temperatures the tolerance is close to zero. This is particularly disadvantageous for ALD processing where the valve operates 100 to 10,000 times during one process. The heat produced by the friction of the actuator piston movement increases quickly because of rapid pulsing. By contrast, in conventional chemical vapor deposition the precursor valve operates only twice during one process.
Thus, as explained in earlier ALD patents and will have become apparent to a person skilled in the art from the above description, a fast acting dosing (pulsing) system with non-wear valves (low-particle level) characteristics would be essential for improved ALD processing.